1. Field of the Invention
The present invention relates to a recording medium capable of recording information and erasing the recorded information in a reversible manner by application of heat or by irradiation with light. By the use of this recording medium according to the present invention, monochrome and multicolor recording can be performed. The medium can be used particularly as an optical recording medium, and applied to a rewritable color recording sheet for a color copying machine, a color printer, and a facsimile, and a rewritable memory card having a display function.
2. Description of the Prior Art
As recording materials for use in a rewritable recording medium, there are two kinds of materials as follows: materials in photon mode such as photochromic materials which change in color in a reversible manner by irradiation with light; and thermosensitive recording materials which bistably change in phase in a reversible manner on heating and cooling. As the latter materials, phase change type recording materials, photomagnetic recording materials, and the like are well known which transform light energy into thermal energy based on their light absorption properties to conduct recording.
Organic materials are widely used for the thermosensitive recording materials. Examples of these materials include (i) thermosensitive dye type recording materials which comprise a leuco dye, a coloring agent, and an agent for erasing a color in combination, and which reversibly develop and erase a color; (ii) phase change type thermosensitive recording materials comprising a matrix polymer and organic crystalline microparticles dispersed therein, the microparticles changing in transparency depending upon the solidifying conditions determined in response to the heating and cooling process of the matrix containing the microparticles; and (iii) thermosensitive recording materials comprising a liquid crystal polymer such as a cholesteric liquid crystal, the transparency of the recording material being changed by changing the molecular orientation of the polymer by application of heat.
Among the thermosensitive recording materials mentioned above, the materials of type ii which comprise a matrix polymer and organic crystalline microparticles therein as mentioned above have attracted attention. In this type of materials, information is recorded and erased in a reversible manner based on a change in transparency of the recording material depending on the solidifying conditions of the microparticles. That is, when the above-mentioned organic crystalline microparticles are melted by application of heat, and then solidified by lowering the temperature, the microparticles achieve various types of solidified states such as a polycrystalline state, a single crystalline state, an amorphous state, or a non-crystalline state, according to the melting and solidifying conditions. Each state has different transparency characteristics. Thus, the recording and erasing of information is accomplished by utilizing this phenomenon.
For example, as shown in FIG. 8b, a sheet made of a transparent matrix polymer 310 comprising organic crystalline microparticles in a polycrystalline crate 311B is opaque as a whole at ordinary temperature since the organic crystalline microparticles scatter light. As shown in FIG. 9, when this polymer sheet is gradually heated and the temperature exceeds T.sub.0 (which is approximately equal to the glass transition point (Tg) of the matrix polymer), the polymer sheet begins to change from en opaque state to a transparent state. When the temperature reaches T.sub.1, the polymer sheet becomes almost completely transparent. At this point, the organic crystalline microparticles 311A (see FIG. 8a) are light transmissible. When the temperature is raised further to T.sub.2 or more, the light transmittance of the organic crystalline microparticles gradually decreases, and the polymer sheet becomes semi-transparent at T.sub.3 (approximately corresponding to the melting point of the organic crystalline microparticles). When the organic crystalline microparticles that were previously heated to a temperature between T.sub.1 and T.sub.2 are cooled to room temperature, the organic crystalline microparticles are light transmissible, and thus the polymer sheet remains transparent. When the organic crystalline microparticles that were previously heated to T.sub.3 or more are cooled to room temperature, the organic crystalline microparticles scatter light, and thus the polymer sheet becomes opaque. Because of this, for example, the state of the transparent sheet which has been cooled to room temperature, after having been heated to a temperature between T.sub.1 and T.sub.2, is made an initial state. Information is written to the sheet at a temperature of T.sub.3 or more, whereby information is recorded. In the alternative, the state in which the sheet is opaque is made an initial state, and information can be written to the sheet so that the sheet becomes transparent.
Japanese Laid-Open Patent Publication No. 54-119377 discloses the combination of the above-mentioned matrix polymer and the organic crystalline microparticles. Examples of the materials for the organic crystalline microparticles disclosed therein include aliphatic and aromatic alcohols, carboxylic acids, amines, and amides, end halides and sulfides thereof. Examples of the matrix polymer disclosed therein include polyesters, polyamides, polyacrylic acid, polystyrene, silicone resins, polyvinyl chloride, polyvinylidene chloride, and polyacrylonitrile. Improved recording materials which further comprise carbon black for transforming light energy into thermal energy and which further comprises additives such as an anti-oxidant are disclosed in Japanese Laid-Open Patent Publication Nos. 57-82087 and 57-82088, respectively.
However, there has been a problem in that none of the conventional recording materials mentioned above can perform rewritable multicolor recording.